osd: special case CALL op to not have RD bit effects

In commit 20496b8d2b2c3779a771695c6f778abbdb66d92a we treat a CALL as
different from a normal "read", but we did not adjust the behavior
determined by the RD bit in the op.  We tried to fix that in
91e941aef9f55425cc12204146f26d79c444cfae, but changing the op code breaks
compatibility, so that was reverted.

Instead, special-case CALL in the helper--the only point in the code that
actually checks for the RD bit.  (And fix one lingering user to use that
helper appropriately.)

Fixes: #3731
Signed-off-by: Sage Weil <sage@inktank.com>